I. Field of the Invention
The present invention relates to wireless communications. More particularly, the present invention relates to pseudo-random number (PN) generators for spread spectrum communications systems.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communication in which a large number of system users are present. Although other multiple access communication system techniques are known in the art, such as time division multiple access (TDMA and GSM), frequency division multiple access (FDMA), and AM modulation schemes such as amplitude companded single sideband (ACSSB), the spread spectrum modulation technique of CDMA has significant advantages over these other modulation techniques for multiple access communications systems. The use of CDMA techniques in a multiple access communications system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d and U.S. Pat. No. 5,103,459, issued Apr. 7, 1992, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d both assigned to the assignee of the present invention and incorporated herein by reference.
CDMA systems are typically designed to conform to a particular CDMA standard. Examples of such CDMA standards include the xe2x80x9cTIA/EIA/IS-95-A Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d (hereinafter, the IS-95-A standard) and the TIA/EIA/IS-98-A, -B, and -C standards entitled xe2x80x9cRecommended Minimum Performance Standard for Dual-Mode Spread Spectrum Cellular and PCS Mobile Stations.xe2x80x9d
CDMA systems are direct sequence spread spectrum systems that spectrally spread the transmitted data over an entire available system bandwidth with a set of inphase and quadrature pseudo-random noise (PN) sequences. The PN sequences are selected because of certain important xe2x80x9crandomnessxe2x80x9d properties that provide superior performance. Each CDMA standard defines the specific PN sequences to be used for spreading the data.
In CDMA systems, a particular geographic area can be partitioned into a number of neighboring cells and each cell can be further partitioned into a number of sectors. Each cell or sector is serviced by a base station that communicates with a number of mobile stations located within or near the cell or sector coverage area. The forward link refers to transmissions from the base station to the mobile station and the reverse link refers to transmissions from the mobile station to the base station. The forward link and the reverse link are allocated separate frequencies.
For a CDMA system that conforms to the IS-95-A standard, each transmitting base station is assigned a particular offset of the PN sequences. Specifically, in accordance with the IS-95-A standard, the base stations are each assigned one of 512 possible offsets. The assigned offset allows the mobile station to identify each base station with which it communicates.
On the forward link, a pilot signal is typically transmitted by the base station and used by the mobile station for acquisition. For an IS-95-A compliant system, the pilot signal is simply a transmission of the PN sequences at the assigned offset. The pilot signal enables the mobile station to acquire a local base station in a timely manner. The mobile station also derives synchronization information and relative signal power information from the received pilot signal.
As demand for wireless communication increases, a geographic area may include multiple spread spectrum systems. For example, the geographic area may be simultaneously serviced by one CDMA system operated at a cellular frequency band and another CDMA system operated at a Personal Communications System (PCS) (or another) frequency band. The mobile station may be designed with the capability to acquire and communicate with one or more CDMA systems. During the acquisition process, if the base stations of these different CDMA systems transmit using similar PN sequences, the mobile station may not be able to easily distinguish the pilot signals from these systems. As a result, additional signal processing may be required to acquire and identify the desired system, which can prolong the acquisition process.
Thus, techniques that aid in the detection and acquisition of a particular CDMA system in a multiple systems environment are highly desirable.
The present invention provides techniques to improve the acquisition process in a spread spectrum environment in which a mobile station receives forward link spread spectrum signals from multiple CDMA systems or in which the mobile station does not have a priori knowledge of the source of the received signal. In accordance with the invention, the spread spectrum signals from different CDMA systems are spread with different sets of PN sequences, with the PN sequences from each set being uncorrelated to the PN sequences in the other sets. By using uncorrelated PN sequences, the likelihood of detecting a pilot signal from an undesired system is reduced or minimized, and the mean time to acquisition of the pilot signal from the desired system is improved.
An embodiment of the invention provides a method for acquiring a particular one of a number of spread spectrum signals, with the particular spread spectrum signal being spread with a particular set of PN sequences. In accordance with the method, a first set of PN sequences is identified and corresponds to a first hypothesis of the particular spread spectrum signal being acquired. A received signal is then processed with the identified set of PN sequences to extract a pilot signal. A metric is computed for the extracted pilot signal and used to determine whether the pilot signal has been acquired. If the pilot signal is determined to not be acquired, a second set of PN sequences corresponding to a second hypothesis of the particular spread spectrum signal is selected and used to process the received signal. The PN sequences in the second set are uncorrelated to the PN sequences in the first set.
In a specific implementation, the PN sequences in the second set are the reverse of the PN sequences in the first set. The PN sequences in the first set can be generated based on the characteristic polynomials defined by the IS-95-A standard.
Another embodiment of the invention provides a method for a receiver unit configurable to acquire a particular one of a number of spread spectrum signals. The receiver unit includes a receiver, a demodulator, a despreader, a PN generator, a processing unit, and a controller. The receiver receives and conditions a received signal, which includes the particular spread spectrum signal, to provide a conditioned signal. The demodulator demodulates the conditioned signal to provide baseband signals, and the despreader despreads the baseband signals with a first set of PN sequences to provide despread signals. The PN generator provides the first set of PN sequences, which is selected from among a number of sets of PN sequences and corresponds to a first hypothesis of the particular spread spectrum signal being acquired. The processing unit processes the despread signals to extract a pilot signal and computes a metric for the extracted pilot signal. The controller determines whether the pilot signal has been acquired based, in part, on the computed metric. If the pilot signal is determined to not be acquired, the controller directs the PN generator to provide a second set of PN sequences corresponding to a second hypothesis of the particular spread spectrum signal being acquired. The PN sequences in the second set are uncorrelated to the PN sequences in the first set.
Again, in a specific implementation, the PN sequences in the second set are the reverse of the PN sequences in the first set. The PN sequences for the first set can also be generated based on the characteristic polynomials defined by the IS-95-A standard.
Yet another embodiment of the invention provides a transmitter that includes a spreader, a PN generator, a modulator, and a transmitter. The spreader receives and spreads pilot data with a set of PN sequences to provide spread pilot data. The PN generator provides the set of PN sequences, which is generated based on the following characteristic polynomials:
P1,2=x15+x10+x8+x7+x6+x2+1, and
PQ,2=x15+x12+x11+x10+x9+x5+x4+x3+1.
The modulator modulates the spread pilot data to provide a modulated signal, and the transmitter receives and conditions the modulated signal to provide a spread spectrum signal. The pilot data can be gated.